Recent research has established that the arm is represented at multiple sites in the motor areas of primate cerebral cortex. In particular, we have discovered four separate representations of major muscles of the arm in the MI motor cortex of the rhesus monkey, with the use of intracortical microstimulation (ICMS) methods. Each representation is biased, however, toward muscles which act principally about a particular joint. We propose, therefore, that each representation is specialized for the control of movements about a particular joint, but that each can also generate the muscle synergies that are needed along the arm for mechanical support of the movement. We propose also that these MI arm areas can be altered by motor experience and by the state of adjacent arm areas, thus participating in motor learning and in recovery from local cortical injury. Finally, we propose that the several arm areas outside of MI are specialized for arm- hand control under specific stimulus and motivational conditions. To test these hypotheses, we will use chronically implanted multi-electrode systems to observe multiunit activity simultaneously in the four arm areas of MI. We will observe the distributions of neural activity across these areas during controlled movements about different joints in the arm, during the learning of these movements, and during recovery from a lesion in an adjacent MI arm area. Repeated ICMS mapping through the same electrodes will be used to detect related changes in the spatial properties of each representation. Finally, we will implant electrodes in several arm-hand areas in addition to MI, and examine the constellations of zones that 'light up' under different stimulus and motivational conditions. These experiments will provide important new observations on the distribution of activity across a distributed motor cortical network during well defined behaviors, with a resolution in both time and space that surpasses that of the best current imaging systems. The results will extend our understanding of the functions of multiple representation in motor systems, and the relations of different motor areas to various implications for programs aimed at developing neurally controlled prostheses for the handicapped.